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If you're planning to have a baby any time soon, chances are they'll live to see their 100th birthday.

A review published in today's edition of the Lancet, predicts half of all babies born in wealthy countries today will live for a century if current life expectancy trends continue.

It also finds that not only will people live longer, but they'll spend those extra years with less disability.

"Life expectancy is increasing in most developed countries, with no signs of deceleration," say the review's authors, led by Dr Kaare Christensen of the University of Southern Denmark.

Over the past 165 years improvements in health care, medicine and lifestyle, as well as decreases in infant mortality have all contributed to increasing longevity, they write.

They say the key question has been whether increased life expectancy is also accompanied by functional limitations and disabilities.

Disease increase

The review points out, that despite improved life expectancies, an increase in cancer, cardiovascular disease and other chronic illnesses has risen with ageing populations.

But they write that most evidence for people aged 85 years or younger suggests disabilities are postponed in older age.

This apparent contradiction can be explained by "early diagnosis, improved treatment and amelioration of prevalent diseases so they are less disabling."

But for people aged older than 85 years, the situation is less clear.

Burden on society

Some research suggests "exceptional longevity" comes with problems not only for the individual but society as a whole," they write.

But, "data for exceptionally old people are few and inconsistent."

A recent Danish study hypothesised that living an extremely long life would result in functional limitations.

But it found the cognitive and physical functions didn't change significantly between those people studied who were 92 and those who were 100, reports the review.

An Australian expert warns most people won't be able to live to 100 without the help of medical technology.

Professor Brian Morris of the University of Sydney says the study is naive to think the majority of people will live to be 100 naturally.

"Even if you have the healthiest lifestyle, including diet and exercise and you don't smoke you're still going to hit a glass ceiling at about 85," he says.

Morris says there is a biological limit that will prevent a lot of people reaching a century without medical and technological help.

Regardless of whether elderly people will be physically and mentally functional in very old age, the review's authors say population ageing will have social, medical and economic implications for society.

"It poses severe challenges for the traditional welfare state."

Changing retirement

They say a key strategy to cope with the economic burden of an ageing population is to extend the age of retirement.

"Improvements in health and functioning along with shifting of employment from jobs that need strength to jobs that need knowledge imply... that people in their 60's and 70's are capable of contributing to the economy."